Batten Down the Hatches!

It took me a while to dig out from the latest problems with the National Weather Service server from which we obtain most of our model data.  So it wasn't until this afternoon that I took a look at the model guidance.

Wow.

I hesitate to throw gas on the inferno of hype currently burning through social media, but on paper, things are looking good.

I like large-scale patterns in which the flow is progressive with "open wave" troughs, and this is what we have coming.  This means the waves are lower amplitude.  Think of them as ripples in the flow rather than North Shore monsters.  They move quickly, but come in succession, and give us moist flow with a strong component across the Wasatch Range.  Ideal for orographic snowfall generation. This is what we have coming over the next six days [defined here as 0000 UTC 12 February (5 PM MST Thursday) through 0000 UTC 18 February (5 PM MST Wednesday)].

Just look at the GFS 5-day time-height below.  Lots of low level moisture, occasional deep moisture, frequent low-level instability, and long periods of southwesterly to northwesterly flow.  Snowfall won't be continues, but there's a great series of storms.  

Translation: STORM CYCLE.

Numbers?  For Salt Lake City, the European ensemble mean precipitation (water equivalent) for the six-day period is about 1.25 inches, with a range of 0.85-2.0 inches.  Those are pretty good numbers for the airport, which averages 1.27 inches for the entire month of February. 

For Alta, the GFS produces 3.97" of precipitation and 71.7" of snow. The NAEFS, shown below, has a mean of almost 6" of precipitation an 100" of snow and ranges of about 3-8" and 50-140" (more on the extreme snowfall in a minute).  

For some climatological context, the average precipitation and snowfall at Alta for a six-day period in February are 1.35 and 16 inches, respectively.

Let's talk about those snowfall numbers.  One reason that they are so high is large snow-to-liquid ratios.  In other words, low-density snow.  The average snow-to-liquid ratio at Alta is about 13:1, which means 13" of snow from an inch of water.  For most of this forecast period, our algorithm is going for values at or above that.  

I've been worried this year about our snow-to-liquid ratios being too high.  They've been too high in bigger storms.  Still using an average of 13:1 gives you some big numbers.  

Regardless, it appears the ingredients are in place for a major storm cycle.  However, for a forecast that goes this far out, it's difficult to go all in and a look at the plumes above shows the biggest of the three forecast storms is the last.  There's greater uncertainty at those lead times.  Thus, a "low expectations are the key to a happy life" forecast for Alta would call for 2.5-5" of water and 33-66" of snow for the six-day period.  I wouldn't be surprised if we came in above that.  I would be surprised if we came in much below that.  

Now is the time for me to add the caveat that forecasts change, you should monitor official forecasts from the National Weather Service, and, if you are considering venturing in the backcountry, monitor official forecasts from the Utah Avalanche Center.  For backcountry recreation, this has all the hallmarks of a period during which the best options may eventually be to "cower in the trees" or watch the Alpine World Championships on Peacock.  Personally, I hope this storm comes through, everyone lays low, and we get a big natural cycle to clean out some of the slide paths that have gotten choked up with pucker trees and spindly aspen in recent years.